1. Field of the Invention
The present invention relates generally to a solid electrolyte fuel cell, and more particularly, to a solid electrolyte fuel cell in which a plurality of flat unit cells are vertically stacked.
2. Description of the Prior Art
Unlike other types of fuel cells, the solid electrolyte fuel cell is free of leakage and does not require refills so that it is expected to be maintenance-free. The solid electrolyte fuel cell is classified, according to the electrolyte property, into the low-temperature type which is operated at a temperature not exceeding 200.degree. C. and the high-temperature type which is operated at a temperature of about 1000.degree. C. The high-temperature type fuel cell can serve as an energy converter with high energy conversion efficiency, and is expected to be applied to a cogeneration system since the quality waste heat thereof can be utilized. Furthermore, this type of fuel cell does not require a catalyst and is versatile in the use of compatible fuel gas.
In order to obtain a large capacity of electricity, the fuel cells are layered or stacked to form a multilayer structure. The high-temperature type fuel cell is generally made of an oxide (a ceramic) which withstands a high temperature of about 1000.degree. C. at which the cell is operated. When such fuel cells are stacked, the material for stacking must also be made of oxides, thereby restricting a method applicable for stacking the fuel cells. At present, under this restriction, there have been developed or proposed solid electrolyte fuel cells of several shapes and structures and methods for stacking such fuel cells in consideration of the producibility and processability of the oxides used.
Conventionally, the shape of the fuel cell is roughly classified into the cylindrical type and the flat type. Methods for stacking the cylindrical fuel cells such as a bundling method (by Westinghouse) have been developed. Further, a new "monolithic" fuel cell has been proposed, and a method for stacking this type of fuel cells has been studied.
As for materials for the components of the fuel cell, YSZ (yttria stabilized zirconia) is conventionally used for the solid electrolyte, an La-Co, La-Cr or La-Mn group oxide for an air electrode, and a cermet which is a mixture of a ceramic (zirconia) and metal (Ni) for a fuel electrode.
However, the conventional cylindrical solid electrolyte fuel cells, which are basically made of oxides and manufactured depending on the producibility and the processability of oxides used as mentioned above, have limitations in performance and reliability. On the other hand, the flat unit cell is not necessarily made of only oxides, and therefore can achieve performance superior to the cylindrical fuel cell. The monolithic fuel cell is not adequate in the electricity generating efficiency as in the case of the cylindrical fuel cell. Further, since the manufacture of the monolithic fuel cell is complicated by having many different processes, improvement is required for the practical use thereof.
A high electricity generating efficiency is expected from the flat unit cell, and a high output can be produced by forming a large-size flat unit cell. However, stacking the flat unit cells is difficult because there arises problems to overcome such as inter-connection of the fuel cell, separation of a fuel gas and an oxidizing gas, and sealing of each component, in addition to the problems relating to the oxides used, such as processability and reliability of oxide materials and performance of the oxide electrodes and conductors.
Further, a large-size flat unit cell is not effective in gas distribution. Referring to FIG. 6, the gas distribution in a conventional flat unit cell will be described. The fuel cell comprises a solid electrolyte 61, and an air electrode 62 and a fuel electrode 63 disposed on the opposite surfaces of the solid electrolyte 61. Platinum leads 67 are connected to the air electrode 62 and the fuel electrode 63, respectively. The fuel cell composed of the solid electrolyte 61, the air electrode 62 and the fuel electrode 63 is supported by a porous support plate 68, and a cylindrical cell housing 65 covers the entire structure. A fuel gas and an oxidizing gas are supplied to and discharged from the fuel cell. In the large-size flat unit cell having such a gas distribution system, in which each gas is supplied from only one side, the gas densities at the inlet and the outlets of each gas are different, resulting in non-uniform and insufficient gas distribution.
The objective of this invention is to provide a solid electrolyte fuel cell formed by vertically stacking flat unit cells, which is highly gastight and has an effective gas flow so as to allow a uniform gas diffusion to every corner, and can generate a large capacity of electricity efficiently and stably with an improved reliability.